Metabolism represents the organic and chemical processes within a cell that are necessary to maintain life. It is also the way that a cell interacs with energy sources, coordinating energy intake, storage, and utilization, ultimately allowing proper cellular growth and function. A key molecule in metabolism is Acetyl-CoA, a molecule sitting at the crossroads of multiple metabolic pathways, while also serving as a substrate for histone acetylation, one of the main epigenetic marks in the genome. Recent studies have demonstrated that fluctuations in availability of Acetyl-CoA can directly impinge on levels of histone acetylation, yet whether the reverse is possible remains unknown. In other words, may acetate from histone marks serve as a reservoir to modulate Acetyl- CoA levels in order to sustain metabolic homeostasis? And if so, can changes in the epigenome affect metabolism due to a direct effect on acetate availability? In this context, environmental stresses such as ionizing radiation (IR) or hypoxia may directly affect the epigenome, in turn affecting its potentil to buffer Acetyl-CoA levels. This hypothesis represents the main focus of this R21/R33 proposal. Specifically, in the first exploratory (R21) phase the intent is to: 1- Develop a pulse-chase assay using stable-isotope tracing to follow crosstalk between the nucleus and the mitochondria; and 2- Determine the role of the NAD-dependent histone deacetylase SIRT6 in mobilizing acetate from chromatin under conditions of environmental stress. These experiments should set the basis to develop the second R33 phase, where the focus is to: 3- Analyze the role of environmental and nutrient stress in modulating this epigenetics-metabolism crosstalk and 4- Determine the in vivo significance for the nucleus as an energy reservoir. All together, these experiments should provide direct evidence for a role of the nucleus as a metabolic reservoir, which may point to strategies where modulation of epigenetic marks may serve as a strategy against metabolic diseases and environmental stresses.